Axial piston pump with rocker cam counterbalance feed

ABSTRACT

A rocker cam type, variable displacement, axial piston pump where working pressure fluid supplied to said housing is fed to a pair of counterbalance pockets formed in the rocker cam through fluid passages located within the rocker cam.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a variable displacement, axial piston pump having a rocker cam pivotally mounted in a rocker cradle formed in the pump housing to thereby permit the displacement of the pump to be changed. Such a pump includes a rotatably mounted pump barrel having a plurality of longitudinal bores each housing a pump piston. Each piston has a shoe pivotally attached to a head end which projects from the barrel and is retained against a thrust surface formed on the rocker cam. A prime mover rotates the barrel causing the pistons to reciprocate and thereby pump fluid as the piston shoes slide over the thrust surface. The angle of the rocker cam thrust surface determines the displacement of the pump. If the rocker cam is centered, i.e. off-stroke, the thrust surface is perpendicular to the axes of the pistons and they do not reciprocate. If the rocker cam is not centered, i.e. on-stroke, the thrust surface is angled with respect to the ends of the shoes and the piston reciprocates as the barrel is rotated.

[0002] The rocker cam may be pivoted to change the displacement of the pump by a rotary servo controlled fluid motor as described in U.S. Pat. No. 3,967,541 assigned to the predecessor in interest to the assignee of the present invention and hereby incorporated by reference. Alternatively, the pump may be put on-stroke by a spring which biases the rocker cam to a full on-stroke position as is common with a pressure compensated pump of the type disclosed in the instant application. One type of pressure compensated pump may be seen in U.S. Pat. No. 4,289,452 which is hereby incorporated by reference herein.

[0003] During the operation of an axial piston pump, forces caused by pumping working pressure fluid are transmitted through the pistons and the thrust surface to the interface between the rocker cam and the rocker cradle. As the pressure of the working fluid increases, the pumping forces applied to the rocker cam/rocker cradle interface increase. In order to overcome the pumping forces at the rocker cam/rocker cradle interfaces, fluid pockets are formed in either the rocker cradle or the rocker cam. Working pressure fluid is supplied through the piston shoes, the thrust surface and the rocker cam to the fluid pockets to provide a counterbalancing force to the pumping forces applied to the rocker cam/rocker cradle interface. The counterbalancing force greatly reduces the force a pressure compensator control or a rotary servo control must be capable of exerting to change the displacement of the pump.

[0004] Because of the complexities involved in balancing the piston shoes, it is desirable to provide a means of supplying working pressure fluid to the rocker cam/cradle interface fluid pockets which does not require feeding the fluid from the piston shoes into the rocker cam.

SUMMARY OF THE INVENTION

[0005] An axial piston pump for pumping fluid at a set working pressure which comprising a housing, a barrel rotatably mounted in the housing, a plurality of bores in the barrel, a piston mounted for reciprocation in each of said bores, a shoe mounted on the end of each piston, a rocker cam support in the housing, a rocker cam having a pair of spaced arcuate bearing surfaces pivotally mounted in complementary arcuate bearing surfaces formed in said rocker cam support, a thrust surface on said rocker cam, a holddown for clamping said piston shoes against said thrust surface wherein said shoes slide over said thrust surface when said barrel is rotated and the pistons reciprocated in their bores when the rocker cam thrust surface is angled to the axis of said barrel, a counterbalance fluid pocket formed in each of said rocker cam arcuate bearing surfaces, a first fluid passage formed in said housing connected to a source of working pressure fluid for conducting working pressure fluid to one of said counterbalance fluid pockets and a second fluid passage formed in said rocker cam connecting said one and the other of said counterbalance fluid pockets such that working pressure fluid supplied to said one counterbalance fluid pocket through said first fluid passage in said housing is supplied to said other counterbalance fluid pocket through said second fluid passage formed in said rocker cam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a perspective view of a rocker cam type pump which incorporates the instant invention looking at the intake and discharge ports;

[0007]FIG. 2 is a side view, partially sectioned of the pump in FIG. 1;

[0008]FIG. 3 is an axial sectional view of the pump taken along line 3-3 of FIG. 2;

[0009]FIG. 4 is a sectional view of the portion of the pump housing having a rocker cradle formed therein;

[0010]FIG. 5 is a perspective view of the pump with the housing removed to illustrate the rocker cam and other internal parts;

[0011]FIG. 6 is a perspective view of a rocker cam; and

[0012]FIG. 7 is a rear view partially in section of the rocker cam depicted in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBOIDMENT

[0013] Referring to FIGS. 1 through 5 of the drawings, a pressure compensated, variable displacement, axial piston pump 10 having a rocker cam pivotally mounted in a cam support or cradle may be seen to include a central housing 12, having a mounting pilot end 14 and a port cap 16 at the other end. Bolts 17 connect port cap 16 to housing 12.

[0014] Housing 12 defines a cavity which houses a rotatable barrel 18 mounted on a drive shaft 20. The inner end of drive shaft 20 is supported in a bearing 22 mounted in the port cap 16. Drive shaft 20 also is supported in a bearing 24 mounted within housing 12 and has a splined drive end 26 which projects outwardly of housing 12.

[0015] Barrel 18 has a plurality of bores 28 equally spaced circumferentially about its rotational axis. Each bore 28 contains a piston 30 having a ball shaped head 32. A shoe 34 is swedged onto head 32 of piston 30 such that the shoe can pivot about the end of the piston. Each of the shoes is clamped against a flat thrust plate or swash plate surface 36 formed on the face of a pivotal rocker cam 38 utilizing a conventional shoe retainer assembly of the type described in detail in U.S. Pat. No. 3,904,318 assigned to the predecessor in interest of the assignee of the subject invention.

[0016] Turning to FIGS. 2 through 6, it may be seen that rocker cam 38 has a pair of arcuate bearing surfaces 40 which are received in complementary arcuate bearing surfaces 42 which comprise a rocker cam support or cradle 44 formed in mounting pilot end 14 in housing 12. Rocker cam 38 pivots about a fixed axis perpendicular to the axis of rotation of barrel 18 to change the displacement of pump 10. In operation, the prime mover, not shown, affixed to spline drive end 26 rotates drive shaft 20 and barrel 18 within housing 12. When thrust surface 36 on rocker cam 38 is perpendicular to the axis of rotation of barrel 18, rotation of barrel 18 will cause the shoes to slide across the surface of thrust surface 36 but no pumping action will occur inasmuch as the pistons 30 will not reciprocate within bores 28. In other words, when thrust surface 36 is perpendicular to the axis of drive shaft 20, the pump is in a position of minimum fluid displacement. As rocker cam 38 and thrust surface 36 are inclined from this position, the pistons 30 will reciprocate within bores 28 as shoes 34 slide over the surface of thrust plate 36. As the pistons 30 move inwardly of bores 28 i.e. away from port plate 46, low pressure fluid is drawn into cylinder bores 28 from inlet port 48. As piston shoes 34 slide across thrust surface 36 and move toward port plate 46, high pressure fluid is expelled through outlet port 50. It should be noted that fluid displacement increases as the angle of inclination of thrust surface 36 increases. Referring to FIG. 3, it may be seen that rocker cam 38 and thrust surface 36 are shown in a position of maximum fluid displacement. Rocker cam 38 may be pivoted clockwise to reduce the displacement of pump 10. Although, pump 10 of the instant embodiment is depicted as a pressure compensated pump which does not cross center, the instant invention described hereinbelow applies equally to a rocker cam type variable displacement axial piston where rocker cam 38 may be pivoted clockwise across center such that the intake and exhaust ports are reversed and the device is providing maximum fluid displacement in the opposite direction. Such a pump may be seen in U.S. Pat. No. 5,076,145 assigned to the predecessor in interest of the subject invention. The instant invention also applies equally to a rocker cam type, variable displacement pump having a rotary servo or linear servo type control.

[0017] In the instant embodiment, in which pump 10 is depicted as a pressure compensated device, a piston 52 is slidably mounted in a bore 54 formed in a cylinder 56 rigidly mounted within port cap 16. A spring 58 around cylinder 56 biases piston 52 against a button 60 mounted on one side of rocker cam 38 to force the rocker cam to pivot to a position of maximum fluid displacement. A stroking piston 62 is slidably mounted in a bore 64 of a cylinder 66 rigidly secured in port cap 16 at a position within pump housing 12 diametrically opposite that of biasing piston 52. Stroking piston 62 engages a button 68 mounted in rocker cam 38 at a position diametrically opposite that of button 60.

[0018] In a pressure compensated pump it is necessary to reduce the displacement of the pump when the pressure of the discharge fluid becomes excessive. When this condition occurs, pressure fluid is supplied to the end of stroking piston 62 to force it to move outwardly of bore 64 and thereby cause rocker cam 38 to pivot clockwise (as viewed in FIG. 3) towards a position of reduced fluid displacement. Stroking piston 62 will continue to pivot rocker cam 38 until such time as the discharge pressure of working fluid falls below a maximum setting. When this occurs, pressure fluid no longer is supplied to stroking piston 62 and biasing spring 58 moves stroking piston 52 outwardly to thereby pivot rocker cam 38 in a counterclockwise direction and thereby increase the displacement of the pump. Inasmuch as the instant invention is for any type of rocker cam type pump independent of its displacement control, a further description of the pressure compensated mechanism of pump 10 is not required.

[0019] As mentioned above, when rocker cam 38 is pivoted counterclockwise sufficiently to cause working pressure fluid to be expelled from pump 10 at a relatively high pressure, large pumping forces are exerted through pistons 30 to rocker cam 38. These forces are transmitted through the complementary arcuate bearing surfaces 40 and 42 into rocker cam support 44. The large pumping forces cause large friction forces to occur at the interface of rocker cam bearing surfaces 40 and rocker support bearing surfaces 42 to make movement of rocker cam 38 within rocker support 44 very difficult. In an attempt to reduce the friction forces between rocker cam 38 and rocker support 44 plane bushings 70 are inserted between rocker cam arcuate bearing surfaces 40 and rocker support arcuate bearing surfaces 42 as depicted in FIG. 2. While plain bushings 70 reduce the aforementioned frictional forces to some extent, they are inadequate by themselves to reduce the frictional forces to a satisfactory level.

[0020] Accordingly, working pressure fluid is supplied to counterbalance pockets 72 and 74 formed in the rear faces 76 of rocker cam 38 as depicted in FIGS. in 2, 5 and 6. The areas of the counterbalance pockets 72 and 74 are designed such that when they receive working pressure fluid they reduce the force required to pivot rocker cam 38 within cam support 44 to within desirable levels. Heretofore, working pressure fluid has been supplied to counterbalance pockets in rocker cam where the working pressure fluid source is a pumping piston and fluid is supplied to the piston shoe and thereafter to bores in the thrust plate which bores connect to the counterbalance pockets.

[0021] Applicant has discovered a unique means for supplying working pressure fluid to the counterbalanced pockets 72 and 74 formed in the rear face 76 of rocker cam 38 where the fluid source is in the housing 12.

[0022] Turning to the FIGS. 2 and 4 through 7, it may be seen that a fluid passage 78 connected to a source, not shown, of working pressure fluid is formed in housing 12. Fluid passage 78 opens into a fluid passage 80 formed in housing 12 one end of which is closed by a plug 82 which may be replaced with a sensor or other device utilizing working pressure fluid for control purposes.

[0023] A hollow roll pin 84 is mounted in a central bore of plain bushing 70, in cam support arcuate bearing surface 42 and in a corresponding bore in housing 12. Roll pin 84 serves two purposes. It anchors plain bushing 70 on cam support or cradle 44 and it intersects fluid passage 80 to thereby connect that passage to a fluid passage 86 formed in rocker cam 38 and in arcuate cam surface 40. Fluid passage 86 intersects an angled fluid passage 88 formed in rocker cam 38 and closed by a plug 90. Fluid passage 88 intersects an oppositedly angled passage 92 closed at one end by a plug 94. The fluid passage 96 which parallels fluid passage 86 has one end which intersects fluid passage 92 at a right angle and another end which opens into fluid pocket 74 formed in rear face 76 of rocker cam 38. Turning to FIG. 2, it may be seen that a roll pin 98 anchors plain bushing 70 to cam support surface 44.

[0024] Thus, it may be seen that the instant invention provides a means of taking source of working pressure fluid in the pump housing and supplying the fluid to the counterbalance pockets 72 and 74 formed in the rear face 76 of rocker cam 38 which pockets are connected through passages formed in rocker cam 38. This greatly simplifies supplying pressure fluid to the counterbalance pockets 72 and 74. 

We claim:
 1. An axial piston pump for pumping fluid at a set working pressure which comprising a housing, a barrel rotatably mounted in the housing, a plurality of bores in the barrel, a piston mounted for reciprocation in each of said bores, a shoe mounted on the end of each piston, a rocker cam support in the housing, a rocker cam having a pair of spaced arcuate bearing surfaces pivotally mounted in complementary arcuate bearing surfaces formed in said rocker cam support, a thrust surface on said rocker cam, a holddown for clamping said piston shoes against said thrust surface wherein said shoes slide over said thrust surface when said barrel is rotated and the pistons reciprocated in their bores when the rocker cam thrust surface is angled to the axis of said barrel, a counterbalance fluid pocket formed in each of said rocker cam arcuate bearing surfaces, a first fluid passage for receiving a source of working pressure fluid formed in said housing, a second fluid passage formed in said housing and connected to said first fluid passage for conducting working pressure fluid to one of said counterbalance fluid pockets and a third fluid passage formed in said rocker cam connecting said one and the other of said counterbalance fluid pockets such that working pressure fluid from said first fluid passage supplied to said one counterbalance fluid pocket through said second fluid passage is supplied to said other counterbalance fluid pocket through said third fluid passage.
 2. The axial piston pump of claim 1 further comprising a pressure compensator control. 